1. Field of the Invention
The present invention relates to an optical coupler which can be used for, e.g., a one-directional large-capacity/bidirectional small-capacity subscription type optical communication system adapting a time-compression, multiplexing technique.
2. Description of the Related Art
Recently, a so-called subscription type optical communication system is under development in a large scale, in which optical fibers are provided in not only a conventional basic trunk network connecting exchanges but also a subscriber network connecting exchanges and buildings so that voices, computer data, images and the like can be simultaneously transmitted at a high speed. An optical transmission system as shown in FIG. 1 is proposed in the form of such a system.
Referring to FIG. 1, reference symbol A denotes an exchange; and B, a subscriber station. First, in the exchange A, reference numeral 1a denotes a large-capacity (e.g., several hundreds of Mbits/sec.) optical transmitter. The optical transmitter 1a outputs an optical signal having a wavelength .lambda.1 to an optical fiber 6 serving as a transmission path between the exchange A and the subscriber station B via an optical multiplexer/demultiplexer 5. Reference numeral 2a denotes a small-capacity (e.g., several tens of Mbits/sec.) optical transmitter which transmits a light wave having a wave-length .lambda.2. Reference numeral 3a denotes a small-capacity optical receiver which receives the light wave having the wavelength .lambda.2. The optical receiver and transmitter 3a and 2a are connected to the optical fiber 6 via a directional coupler 4 and the optical multiplexer/demultiplexer 5.
The subscriber station B opposing the exchange A has a large-capacity optical receiver 1b, a small-capacity optical receiver 2b, and a small-capacity optical transmitter 3b. The light waves having wavelengths .lambda.1 and .lambda.2 which are transmitted through the optical fiber 6 are demultiplexed by the optical multiplexer/demultiplexer 7. The light wave having the wavelength .lambda.1 is received by the optical receiver 1b. The light wave having the wavelength .lambda.2 is received by the optical receiver 2b via a directional coupler 8. The small-capacity transmission with the wavelength .lambda.2 is so-called ping-pong transmission in which transmission and reception are performed bidirectionally in a time-divisional manner. For this purpose, this system uses the directional couplers 4 and 8.
In the optical transmission system described above, the wavelength .lambda.1 is assigned to large-capacity information transmission such as TV distribution services, and the wavelength .lambda.2 is assigned to small-capacity information transmission such as telephone services. Then, two different services can be independently transmitted by using a single transmission path optical fiber.
However, the optical transmission system described above has the following drawbacks as each station uses an optical multiplexer/demultiplexer and a directional coupler.
(1) The optical multiplexer/demultiplexer and the directional coupler must be connected, and this connection is time-consuming and requires much labor.
(2) A connection loss occurs at the connecting portion of the respective optical components with the optical fiber.
(3) As the number of optical components is large, the system becomes large. The place where the system can be installed is limited especially when the system is to be installed in a building or house.